Hey guys, i've been working on a project in the background, a modular PDA-style device that would run mainline linux and act as a sort of hackable smartphone for embedded linux devs who want to tinker (or cosplayers who need a really weird sort of HMI, whoever). The actual embedded side isn't really an issue for me so far, the issue that seems difficult to solve is actually that of the connectors...
You see ideally i'd want the device to be fairly thin (so btob mezzanines are a no go, they stack up height quite quickly), but also modular to be able to be adapted to different projects for reuse.
It's basically comprised of three modules, a "core" that has the linux SIP and the power stage required to power it, plus some basic memory. That core is the reusable bit here. It directly connects the 40 pin display interface and mipi-csi camera since it wasn't realistic to route those out to edge connectors for module use (already somewhat crowded).
The top and bottom board "specialize" that core into being a smartphone like device (4G LTE module, wifi/bt modem, BMS and codecs for the bottom, optional EMMC + lux / distance sensing and ear speaker codec for the top) Those modules aren't really routed yet and more in the blockout phase.
Back to the connector issue, to interconnect these 3 modules and allow device expansion on the sides as well (for extra radios or others, add two outer modules), i have to route a variety of signals (EMMC 5, I2S, SDIO, Uart, I2C, USB HS, Can, RGMII... +power) out to the edges of the core module, and somehow get that done in a way that doesn't interfere with the top and bottom cam/display FPCs (so full "fat" connection options like PCIE style edge connectors are somewhat difficult to implement here... small connectors placed on the corners of the board fit best).
So far i've found two possible solutions here, both with serious issues:
going with the edgeboard usb-cs you see here, that gives me 24 straight pins per connection, with a high current rating, few "stubs" in the connector geometry, an outer shield to deal with the sensitivity of single ended signals, and it's a generally very widely available connector type that can be had for cheap, it would be fairly easy for people to spin up their own modules. (the usb-cs on the core are male to avoid people randomly plugging wires into it).
a few issues with this approach:
- horrible manufacturability, if the usb-cs are even slighly misaligned during the assembly step shear stresses will absolutely tear off the pads
- any shock to the device will also cause solder cracks
or, if i go with the alternate approach, quad low profile hirose 12 pin FPCs to match the density, at 0.5mm pitch, i get a soft connection with much simpler mechanical constraints, but it does have its own issues:
- exposed FPC without shielding
- terrible to get power between the boards, we don't get the free lunch there we had with straight usb-c (max 0.5A per pin)
- doesn't provide any alignement certainty, so it's the reverse issue compared to usb-c, we have to have the case of the device handle it all
- low mating count, these things are very fragile and them breaking is basically excpected
- i have to put 4x of them per side, one on the top, one on the bottom, it'll make module connection very fiddly.
- their internal structure is dual sided, which creates an unused "hook" on one side, so it adds a stub to the copper, even if small.
Of course i've also thought about some alternative options like pogo based connectors (wouldn't be great for signal integrity), and custom FPCs with mezzanines on the actual FPCs (quite costly to implement for a device with a 70$ BOM currently)... even if that last one is what basically all phone manufacturers do. It still retains a few of the issues from the off the shelf FPC approach.
Any thoughts on this? I could use some input to figure out if there even is a good solution to this or if i'm tunneling on an XY problem.
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